The lowers for 5.56 (AR 15) and .308 (AR10), are different sizes. You cannot put a .308 upper on a 5.56 lower, or vice versa. You can however build a 7.62x39, 5.45x39, 9mm, 300Blk, 6.5, 6.8, 22lr, and some others with the ar15 lower, hence the multi Cal stamp.